1. Field of the Invention
This invention relates to a method of producing a soil enrichment product from dewatered sludge utilizing a vacuum-auger dryer.
2. Brief Description of the Related Art
Vacuum-auger dryers have appeared in at least two references: U.S. Pat. No. 4,753,016 and U.S. Pat. No. 5,570,516. U.S. Pat. No. 4,753,016 shows a condensation process and apparatus for water vapor that is under vacuum. As explained in the specification, this device appears to have been created for an industrial process such as drying plastic granules as explained in column 3, lines 40-42. U.S. Pat. No. 5,570,516 also appears to be directed to a drying system for an industrial process since the dryer is coupled to an industrial mixer as is shown illustrated in FIG. 2 and as described in the abstract.
These prior art driers apparently have industrial process applications. There has been no known effort made in the field of wastewater treatment to provide a similar method and apparatus for treating a particular by-product of wastewater treatment plants. Accordingly, a need exists to convert dewatered sludge into a soil enriching product such as a loam for fertilizer or top soil having carbon and nitrogen products that have not been reduced to ash which would not be particularly advantageous to assisting soil or grow vegetation.
In the process of treating raw wastewater, it is normally sent to primary tanks where heavier solids settle and are collected for additional treatment and/or disposal. The liquid is transferred from the tanks, aerated and aerobically treated so that the particulates settle out in the secondary settling process as sludge. The water may be removed from both primary and secondary sludges and this by-product is known as dewatered sludge. Normally this unpasteurized dewatered sludge is hauled to landfills where it takes up space. This costs municipalities a relatively large amount in transportation costs. Furthermore, it fills in a landfill which could be utilized for other waste products.
In order to reduce the costs associated with transporting dewatered sludges, it has been discovered that drier sludges have reduced transportation and disposal costs since moisture adds to the weight of the dewatered sludge. In order to remove moisture content the dewatered sludge is typically dried at a high temperature well over 250° F. At high temperatures, some of the dewatered sludge may be burned or otherwise oxidized so that the carbon components in the sludge forms an ash which provides limited, if any, soil enrichment properties. The ash also presents an offensive “burned coffee” odor and can give off particulate emissions that require air pollution controls. Nitrogen components may also be oxidized at a high temperature resulting in a loss of fertilizer value and possibly creating exhaust gasses which may require more extensive pollution controls. Furthermore, most high temperature drying processes pose hazards to operating personnel and may require special operator certification.
Accordingly, a need exists to convert the unpasteurized dewatered sludge into a useful product.